This proposal, in response to RFA-FDA-HFD-79-4 seeks to study the bioavailability and pharmacokinetics of oral chloramphenicol and dicloxacillin in infants and newborns. Chloramphenicol serves as a prototype for an antibiotic metabolized by the liver; dicloxacillin is a prototype for antibiotics whose clearance is highly dependent on renal elimination. During the first three months of life, factors affecting gastrointestinal absorption (gastric pH, gastric emptying time, active transport, pancreatic lipase activity), hepatic metabolism and renal clearance change dramatically. These changes can markedly influence antibiotic bioavailability and pharmacokinetics. Oral antibiotics are used increasingly in infants and newborns for the treatment of minor infections and for continuation of therapy for resolving serious infections. Chloramphenicol and dicloxacillin are two antibiotics commonly administered by the oral route. Chloramphenicol has assumed a major role in the therapy of H. influenzae infections in infants and gram negative enteric infections in the newborn; dicloxacillin has assumed a similar role in the treatment of staphylocaccal infections in the newborn. In spite of the widespread use, there are no reliable bioavailability or pharmacokinetic studies of these antibiotics in this age group. Patients being treated with chloramphenicol or dicloxacillin will be studied during the intravenous phase of therapy; the apparent volume of distribution, elimination constant., clearance, time to peak, magnitude of peak, and area under the curve will be determined. During the oral phase of therapy at steady state, time to peak, magnitude of peak, and area under the curve will be determined. The area under the curve from the intravenous and oral studies will be compared to determine bioavailability. The effects of gestational age, and certain aspects of gastrointestinal physiology on antibiotic bioavailability will be studied. The ultimate goal is to develop dosage guidelines essential for rational use of these antibiotics in infants and newborns.